DL.com is the absolute worst website (yes, I know I'm preaching to the choir). I tried to book an A fare MSY-LAS and use a credit voucher as partial payment. The website kept telling me the fare had changed (almost tripled ) after I hit purchase. Tried every reasonable set of flights and got the same message--even though the site was still touting $471.10 at purchase, each fare had just changed to a minimum of $1100. So, I decided to try one more time. Cleared my cookies etc. Did NOT try to use the voucher. Lo and Behold...I could get the flights I wanted for $471.10. Called the DIamond line to complain. The agent was sympathetic, but, said there was nothing he could do. Really, really annoying . Next up--TTU.